/**
 * Human time helpers for the alarm surfaces.
 *
 * The takeover shows a count-up ("2s ago") rather than a server timestamp: on a
 * lock screen at arm's length an RFC3339 string is unreadable, and the number
 * that matters is how long the phone has been screaming. The absolute time is
 * still available as the accessibility label.
 */

/** Parse a server RFC3339 string. Returns null when absent or unparseable. */
export function parseServerTime(value: string | null | undefined): Date | null {
  if (!value) return null;
  const ms = Date.parse(value);
  return Number.isNaN(ms) ? null : new Date(ms);
}

const SECOND = 1000;
const MINUTE = 60 * SECOND;
const HOUR = 60 * MINUTE;
const DAY = 24 * HOUR;

/**
 * Compact elapsed time. Deliberately coarse above a minute: the takeover updates
 * every second only while the alarm is fresh, and "13m ago" is more useful than
 * "13m 42s ago" on a wall of red.
 */
export function relativeTime(from: Date, now: number): string {
  const elapsed = now - from.getTime();
  // A clock skew between phone and server must not render "in 3 seconds".
  if (elapsed < SECOND) return 'just now';
  if (elapsed < MINUTE) return `${Math.floor(elapsed / SECOND)}s ago`;
  if (elapsed < HOUR) return `${Math.floor(elapsed / MINUTE)}m ago`;
  if (elapsed < DAY) return `${Math.floor(elapsed / HOUR)}h ago`;
  return `${Math.floor(elapsed / DAY)}d ago`;
}

/** Convenience: relative time for a server string, or null when unparseable. */
export function relativeSince(value: string | null | undefined, now: number): string | null {
  const date = parseServerTime(value);
  return date ? relativeTime(date, now) : null;
}

/**
 * Time left before an alert self-destructs, e.g. `Ends in 6d 4h` / `Ends in
 * 12m`. Null when there is no deadline or it cannot be parsed; a deadline in
 * the past renders `Ended` (the service will 404 from then on).
 */
export function remainingLifetime(expiresAt: string | null | undefined, now: number): string | null {
  const date = parseServerTime(expiresAt);
  if (!date) return null;
  const left = date.getTime() - now;
  if (left <= 0) return 'Ended';
  if (left < MINUTE) return 'Ends in under a minute';
  if (left < HOUR) return `Ends in ${Math.floor(left / MINUTE)}m`;
  if (left < DAY) {
    const hours = Math.floor(left / HOUR);
    const minutes = Math.floor((left % HOUR) / MINUTE);
    return minutes ? `Ends in ${hours}h ${minutes}m` : `Ends in ${hours}h`;
  }
  const days = Math.floor(left / DAY);
  const hours = Math.floor((left % DAY) / HOUR);
  return hours ? `Ends in ${days}d ${hours}h` : `Ends in ${days}d`;
}

/** True when the deadline is present and already past. */
export function isPastDeadline(expiresAt: string | null | undefined, now: number): boolean {
  const date = parseServerTime(expiresAt);
  return !!date && date.getTime() <= now;
}

/** Local wall-clock time for the accessibility label of a relative stamp. */
export function absoluteLocal(date: Date): string {
  const hh = String(date.getHours()).padStart(2, '0');
  const mm = String(date.getMinutes()).padStart(2, '0');
  const ss = String(date.getSeconds()).padStart(2, '0');
  return `${hh}:${mm}:${ss}`;
}